Two mutually orthogonally polarized modes are able to propagate in single mode symmetrical dielectric waveguides. When the cross-section of the waveguide is perfectly circular, the two modes maintain the same phase. Thus, no differences in phase occurs between the two waves, allowing the wave to maintain its original polarity.
Optical fibers {dielectric waveguides}having cores of elliptical cross-section are well known to have good polarization holding properties when the difference between the refractive indices of the fiber core and cladding are relatively large. This large difference in refractive index between core and cladding maximizes the difference between the propagation constants of the two fundamental modes of a signal propagated along the major and minor axes of the elliptical cross-section and also minimizes the coupling of the two fundamental modes. This results in reliable pickup of an optical wave signal band through the fiber in the fundamental mode having an electric field parallel to the major axis of the ellipse at the opposite end of the fiber.
In practice however, it is difficult to attain perfect alignment of the wave source and the major axis of the fiber. Instead of single mode propagation along the major axis of the ellipse, the wave is propagated in two orthogonally polarized fundamental modes aligned with both the major and minor axes of the ellipse. The elliptical configuration of the optical fiber tends to hold these two fundamental modes in alignment with these axes throughout the length of the fiber.
It is also known, as shown in U.S. Pat. No. 4,307,938 (Dyott), that unwanted higher order modes are cut off in the region of maximum difference between the orthogonally polarized fundamental propagation modes in elliptical cored optical fibers. Even accidental coupling between the two fundamental modes will reduce the useful band width of the fiber or wave guide. A fiber having a core of elliptical cross-section could be provided in which there was excellent preservation of the polarization of the transmitted signal through the fiber in the operating region in which high order modes of the signal are cut off, but the group velocities of the two fundamental modes of the transmitted signal were equal. This equality of group velocities renders insignificant any disturbances which otherwise would occur from accidental coupling of the two fundamental modes.
Such polarization-maintaining fibers are useful for making physical measurements by the interference of two coherent beams of light; for fiber sensors in which the difference in propagation of one of the orthogonal polarizations is subjected to a property being measured, such as a magnetic field or pressure, and compared to another reference fiber beam; or in heterodyne communications the transmission of signals separately on each of the orthogonal modes in order to increase signal carrying capacity.
The benefits of maximizing the geometrical portion of the birefringence of light by use of a non-circular core shape as discussed above may alternatively be obtained by maximizing the material birefringence obtained by inducing strain in the fiber.
It is known that optical fibers with non-circular cores are much more sensitive to pressure than other optical fibers. This invention addresses the problem of isolating non-circular optical fibers from such pressure and at the same time provides a means to know the orientation of the axis of asymmetry of the optical fiber at the center of the cable from an indicator asymmetry of the outside of the cable.